Time-division multiplexing is commonly employed in the switching fabrics of modem communications switching systems. An illustrative example is the TDM bus of the Lucent Technologies Inc. Definity.RTM. private branch exchange (PBX). New applications and technology quickly exhaust the bus capacity of such existing TDM systems. For example, multimedia communications use large numbers of time slots per line-port or trunk-port, while new hardware technology allows ever-more ports to be implemented via each port circuit pack. It is therefore necessary to increase the bus capacity of the TDM systems.
One approach to increasing the TDM bus capacity runs the bus faster, that is, at a higher clock rate. This approach requires a new design for all circuit packs that connect to and use the TDM bus. This translates to large development and upgrade costs. Hence, this approach is suitable for use only in new system designs where compatibility with existing systems is not required.
A second approach to increasing the TDM bus capacity adds a higher-speed multi-phase clock distribution line to the existing TDM bus. This approach allows existing circuit packs to use the existing clock for conventional operation, and allows new circuit packs to use the new multiphase clock for sub-time-slot operation wherein multiple sub-time-slot transfers occur in a single time-slot. An illustrative example of such an approach is described in U.S. Pat. No. 4,656,627. This approach requires new bus backplanes and clock-generation circuitry, which makes upgrades of existing systems difficult and expensive. Therefore, what the art still lacks is a way to increase the bus capacity of TDM systems that is easily retrofitted into existing systems and does not interfere with the operation of, or require changes to, existing port circuit packs and bus backplanes.